This invention relates to brush seals of the type having one or a pair of backing plates with bristles extending therefrom for sealing engagement with another component and, more particularly, relates to brush seal segments having end plates for protecting the end bristles from damage during handling while maintaining substantially uniform flexibility of the bristles over the length of the brush seal segment.
Brush seals for power generation equipment such as turbines are typically of a segmented design to facilitate assembly. For example, arcuate brush seal segments conventionally comprise one or a pair of backing plates with bristles extending along a face of the one plate or between the pair of backing plates. For ease of description, the invention is described in relation to a brush seal segment having two backing plates with bristles therebetween, although the description is also intended to embrace a brush seal with only one backing plate unless otherwise noted. The bristles have tips projecting beyond the backing plates and into engagement with an adjacent component. In most applications, the arcuate segments are assembled in a circumferential array of, for example, four or six segments, to completely surround, or lie within, an adjacent component, typically a rotating shaft or sleeve. Because the brush seals are generally employed between regions at different pressures, the backing plate on the low pressure downstream side typically has a greater radial extent toward the bristle tips than the backing plate on the higher pressure upstream side. The bristles conventionally lie between the backing plates, secured to one component, and may extend radially, but preferably extend at a skewed radial angle in the direction of rotation of the other component. The opposite ends of the arcuate segments have end faces at substantially the same skewed radial angle as the bristles. The bristles are secured in the arcuate seal segment by, for example, welding along the outer diameter of the arcuate seal segments and along proximal ends of the bristles between the backing plates such that the bristles extend inwardly at a skew angle to engage the rotating component.
It will be appreciated that the bristles at the ends of each segment can be bent or damaged during handling and may tend to flare out from between the backing plates, potentially becoming trapped between adjacent segments during the assembly process. These end bristles are thus easily deflected in either or both circumferential and axial directions. If the bristles bend over the backing plates, the adjacent seal segments, when assembled, trap the bristles between the end faces. Those trapped bristles may cause the segments to xe2x80x9carch-bindxe2x80x9d the entire seal assembly. By this is meant that the trapped bristles force all of the segments away from the component in which the bristles are to form the seal. For example, if the brush seal is disposed about a rotating shaft, the arcuate segments may be forced radially outwardly by the trapped bristles to increase the overall diameter of the seal and hence open up the radial clearance of the rotor. Leakage past the seal is thus increased in addition to the increased leakage that results from the damaged bristles in the first instance.
Further, it is important to minimize the heating effect caused by frictional contact between the brush seal bristles and the rotating component. For example, in steam turbine rotors, unequal heating in a circumferential direction about the rotor may cause sufficient stress in the rotor to bow the rotor. Rotor bowing is exacerbated if there is a proud portion on the rotor which is differentially heated by contact with the brush bristles. In a prior brush seal design described in U.S. Pat. No. 5,110,033, an end tab provided on one of the backing plates at each end of the arcuate segment extends radially to a diameter of the opposed backing plate. The end tab, backing plates and bristles at the segment ends are welded, i.e., fused to one another. It is important, however, that flexibility of the bristles be substantially uniform throughout the circumferential extent of the arcuate segment. If there is differential flexibility, i.e., some bristles being stiffer than other bristles, the stiffer bristles will cause excessive heat to be generated through local frictional contact with the rotating component which can lead to rotor bowing and an unbalanced rotor, with consequent deleterious effects. Because the end bristles adjacent the ends of the backing plates are welded to one another and to the backing plates in the aforementioned patent, the end bristles at opposite ends of these segments are considerably stiffer than the bristles intermediate the end bristles, i.e., the bristles extant between opposite ends of the seal segment. Thus, adverse and differential heating effects in the rotor arise as a result of variations in flexibility of the bristles along the circumferential extent of the seal. In U.S. Pat. No. 5,090,710, a plate is secured across the end of a seal segment by brazing or welding, and extends to the edge of the backing plates. The bristles at the ends of the segment are therefore less flexible and stiffer than those bristles intermediate the segment ends, with similar adverse effects as noted above. Accordingly, there is need for an arcuate brush seal segment to protect the end bristles during manufacture and handling and, additionally, when affording that protection, to provide substantially uniformly flexible bristles throughout the circumferential extent of the seal.
In an embodiment of the invention, a brush seal segment end bristle protection and flexibility maintenance device is provided to protect the end bristles and also maintain their flexibility substantially throughout the entire length of the segments. To accomplish this, the end faces of each arcuate segment are each provided with an end plate or shim which terminates intermediate the edges of the backing plates at a location sufficient to eliminate or minimize the potential for damage to the bristles during handling, while simultaneously enabling the end bristles to extend substantially freely from the backing plates in a manner similar to the remaining bristles intermediate opposite ends of the segment. The end plates are secured to the backing plates. If securement to the bristles is also desired, the end plates may be secured to the bristles at a location remote from the bristle tips. For example, and in conjunction with arcuate brush seal segments for sealing about a rotating shaft, each end plate extends intermediate, preferably about medially, between the inner diameters of the upstream and downstream backing plates, confining the end bristles between the end plates and the backing plates, preventing damage to the bristles during handling. With the end plates secured only to the backing plates or to the backing plates and portions of the bristles remote from the bristle tips, i.e., radially outwardly of a pinch point, the majority of the lengths of the end bristles remain substantially free from the pinch point radially inwardly. As a consequence, flexibility of all of the bristles in the arcuate segment, including the end bristles, remain substantially the same, thereby avoiding one or more groups of bristles which may be stiffer than other bristles and avoiding the adverse heating effects from bristles having non-uniform stiffness. Thus, the problems of flared-out bristles, as well as arch-bound segments and increased stiffness of the bristles, are substantially cured by the foregoing described brush seal.
In a preferred embodiment of the invention, a brush seal comprises an arcuate brush seal segment having at least one backing plate and a plurality of bristles extending along a face of the one backing plate. The bristles are secured to the backing plates adjacent proximal ends thereof and project from an arcuate edge of the backing plate to terminate in bristle tips for forming a seal with an adjacent component. The bristles extend freely from adjacent the proximal ends thereof and an end plate overlying an end of the segment and end bristles thereon. The end plate is secured to the backing plate such that the end bristles and bristles spaced from the end bristles along the segment have substantially the same flexibility.
In a further preferred embodiment of the invention, a brush seal comprises an arcuate brush seal segment having a pair of arcuate backing plates and a plurality of bristles extending between the plates. The bristles project beyond arcuate edges of the backing plates to extend freely therefrom, and terminate at their distal ends in bristle tips for affording a seal with an adjacent component. At least one end plate overlies an end of the segment and end bristles thereof. The plate is secured to end faces of the backing plates such that the end bristles and bristles spaced along the segment from the end bristles have substantially the same flexibility.
In a still further preferred embodiment of the invention, in an arcuate brush seal segment having a pair of arcuate backing plates and a plurality of bristles extending between the plates, the bristles projecting beyond arcuate edges of the backing plates to extend freely therefrom and terminating at their distal ends in bristle tips for affording a seal with an adjacent component, a method of forming the segment comprises the step of securing at least one end plate to end faces of the backing plates such that the bristles spaced along the entire length of the segment have substantially the same flexibility.